The object of this invention is to provide improvements in the construction of cold metal rolling mills, with the purposes of improving their production capability, and improving the flatness of the rolled product.
The earliest known form of rolling mill is the two high mill. This type of mill is still used successfully in some areas, notably where the material to be rolled is relatively soft, as in hot rolling. When harder materials are to be rolled, such as cold metals, the two high mill has become virtually obsolete, since it is not possible to design a two high mill having roll neck bearings of sufficient strength to withstand the roll separating forces in cold rolling.
For this reason the four high mill was developed. In this case relatively small work rolls are used to minimize the roll separating force, and large back-up rolls mounted in large bearings are used to support the work rolls. In order to minimize the work roll diameter (and so to maximize the reducing capability of the mill) the back-up rolls are sometimes driven, and the work rolls non driven. Even so, there is always a practical upper limit to the length to diameter ratio of the work roll, since the work roll is only fully supported in a vertical plane through its axis, and in the horizontal plane through said axis it is only supported at the ends.
The cluster mill (Rohn-U.S. Pat. No. 2,085,449, Sendzimir-U.S. Pat. Nos. 2,169.711; 2,187,250 and 2,776,586) was originally conceived to overcome the limitations of the four high mill, by providing full support of the work rolls in both vertical and horizontal planes throughout their length. The Sendzimir version of the cluster mill also provided virtually uniform support of intermediate rolls by means of casters (eliminating backing roll deflection as a source of flatness error). Several hundred Sendzimir mills have been installed around the world, since for rolling of tough materials such as stainless and alloy steels, high carbon steels, nickel and cobalt alloys and such, the small work roll diameter is necessary.
However, for many materials, particularly for the more common metals such as low carbon steel, copper, brasses, bronzes and aluminum and its alloys, the small work roll of the Sendzimir mill is not necessary for economical operation. Moreover, the small work roll can actually impose a limit on production, due to the greater difficulty of maintaining thermal equilibrium of a small work roll at high speeds then would be the case with a large work roll. On the other hand, the large work roll of the four high mill becomes very inefficient at light gauges due to flattening of said work roll against the surface of the rolled product.
Unfortunately, there is a practical lower limit to the length to diameter ratio of Sendzimir mill work rolls. If the work rolls become substantially larger than the intermediate rolls, spalling and even fracture of said intermediate rolls can result, and if the geometry is modified to use larger intermediate rolls, then the cluster can become unstable.
In practice, for a 72 inch wide rolling mill, the Sendzimir mill has a maximum work roll diameter of about 4 inches. A four high mill, on the other hand of the same 72 inch width, would have a minimum work roll diameter of about 20 inches. For narrower mills, the equivalent diameters of both Sendzimir and four high mill work rolls would be proportionately lower.
Clearly a rolling mill configuration which could be used for work roll sizes in the range 4-20 inches (for a 72" wide mill-proportionately smaller for narrower mills) would potentially be capable of combining the advantages of both 20-Hi cluster mills and four high mills. If such a mill design were available, it would then be possible to establish theoretically the optimum work roll diameter for a given material and thickness range, and then select the mill configuation which would provide this diameter. This would be a big improvement on the present state of the art, where it is usually necessary to choose a work roll diameter either larger or smaller than the optimum, depending on the limitations of the chosen roll configuration.
The present invention discloses a new cluster mill arrangement which can incorporate work roll sizes in the range between existing cluster mill roll sizes and four high mill roll sizes. This arrangement shares the basic characteristic of all Sendzimir cluster mills in that all rolls in the cluster are supported in both horizontal and vertical planes throughout their length.
The present invention also discloses how the optimum work roll diameter for a given material is established, and how this diameter can be achieved using a mill according to the present invention, which thereby represents an improvement on prior art mills for which this optimization cannot, in general be achieved.